1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission, in particular to a hydraulic control device controlling four forward speeds by two solenoid valves.
2. Description of the Prior Art
As shown in Japanese Laid Open Patent No. Sho-63-13949, the applicant of the present invention has proposed a hydraulic control device for an automatic transmission attaining five or four forward speeds.
The automatic transmission has a planetary gear unit having a single planetary gear and a dual planetary gear, wherein the planetary gear unit uses a sun gear commonly for both single and dual planetary gears, and uses a carrier commonly for both single and dual planetary gears. And an output rotation is taken out from the common carrier. A forward first speed is obtained when transmission power is transferred from an input shaft to a ring gear (a small ring gear) of the single planetary gear, and a ring gear (a big ring gear) of the dual planetary gear is restrained; a forward second speed is obtained when the sun gear is restrained under the condition that the transmission power is still transferred to the ring gear of the single planetary gear; a forward third speed is obtained by rotating whole gear unit together; a reverse speed is obtained when transmission power is transferred from the input shaft to the sun gear, and the ring gear of the dual planetary gear is restrained. Due to the above arrangement, an automatic transmission attaining four or five forward speed modes is provided by the combination of a three or four speed automatic transmission mechanism and an under drive mechanism which changes an under-drive with a direct-drive.
Thus, parts and assembly lines for the four and five speed mode automatic transmission are commonly utilized to cope with the wide variation of the car types.
With reference to a hydraulic control device, for common utilization of the above four and five speed mode transmission, each shift valve is switched by a first, a second, a third solenoid valves to attain the forward four or five speed modes.
Furthermore, the following functions are incorporated: when a car stops under a D range state, occurrence of creeping is prevented; an idling control is made, which engages a forward clutch upon receiving signal for the first speed mode a reverse shift control is made, which prevents shift mode from coming into reverse mode even if the shift lever is shifted to the reverse range when running forward.
Though the above mentioned hydraulic control device is suitable for the five speed mode automatic transmission, in case that the device is used for the four speed mode transmission, it is not preferable to employ three expensive solenoid valves by considering production cost and the like. Under such circumstance, it is required to obtain the four speed mode automatic transmission with employing two solenoid valves.